1. Field of the Invention
The present invention relates to a ball-androller bearing. More particularly, the invention relates to a long-lived ball-and-roller bearings needed for an alternator, an intermediate pulley, and an electromagnetic clutch, which are used under clean lubricating condition in an electric motor, automobile or the like.
2. Related Background Art
Traditionally, to implement the longevity of the ball-and-roller bearing, there has been adapted a method to perform a high level cleaning of the bearing material and the use of case hardening steel.
Also, as disclosed in Japanese Patent LaidOpen Application No. 62-132031, there is a conventional example in which the carburizing hardening layers of a bearing ring and rolling element are formed deeper to prolong the bearing life.
Futhermore, as disclosed in Japanese Patent Publication No. 62-29487, there is a conventional example in which the bearing cracking tenacity is enhanced by giving an isothermal treatment such as a martempering to a bearing steel of SUJ 3 and the like, for example, so that the life of the bearing is prolonged.
There is also a conventional example in which attention is given to the compression residual stress which is effective to the anti-rolling fatigue of a ball-and-roller bearing to improve the longevity of the bearing. In this conventional example, the compression residual stress (Sr) works in the direction towards the reduction of the maximum static shearing stress (.tau. max), and the maximum shearing stress corrected by Sr (.tau. max) r (where .tau. max is an absolute value representation) is expressed as given below. EQU (.tau. max) r=.tau. max-1/2(Sr) (1)
(Zaretsky. E. V, NASA TND - 2664; 1965) Here, the greater the compression-residual stress (Sr), the further is reduced the maximum shearing stress ((.tau. max)r). Also, the rolling fatigue life (Life) is expressed as EQU Life=K [1/(.tau. max) r].sup.9 ( 2)
(where K is a proportional constant) so as to intend the improvement of the rolling fatigue life (Life) by adding the compression residual stress (Sr).
However, there has occurred a problem that with the conventional techniques of the high level cleaning used for the bearing steel and the carburization of the bearing material alone, the longevity of the bearings cannot be attained to meet the heavy-load and high-velocity requirements in using the bearings in recent years.
Also, in the conventional example in which attention is given to the compression residual stress, there has been a problem that even if the compression residual stress is increased, it is impossible to obtain in practice the rolling life expected by the rule to rise to the ninth power expressed in the aforesaid equation (2) and the longevity improvement factor is rather small.
Also, the conventional example disclosed in the Japanese Patent Laid-Open Application No. 62-132031 required a longer time for its heat treatment to form a deeper carburized layer while a specially arranged isothermal treatment is required to implement the conventional example disclosed in the Japanese Patent Publication No. 62-29487. As a result, both of them have a problem in that the heat treatment productivity is low and that a more sufficient life should be obtained in order to meet the desired heavy-load and highvelocity requirements.